214

FLUIDITY AND PLASTICITY

same as that of the single pore?" It is easy to calculate from
Poiseuille's law that for a given area of pore opening the volume
of flow will be directly proportional to the square of the radius of
the individual pores, which are assumed to be alike. If the small
pores have a diameter which is only 0.0001 that of the large one,
the flow which takes place through the large pore in 1 minute
will require about 12 years through the multitude of pores having
the same total area. The underlying principle on which the
explanation is based is the fact that each layer in viscous flow is
carried along by the layer immediately below it, the velocities of
the layers increasing in arithmetical progression. The laws of
viscous flow are therefore capable of explaining why fluids do
not readily flow through jellies and other finely-divided materials.
It is well known that compact clay is almost impervious to
both water and oils, and therefore they are often associated, the
clay forming an impervious stratum through which the oil or water
do not penetrate. The subject of pore openings is therefore
fundamentally important to the subject of the circulation of
water through soils as well as of their retention of water. The
use of compact clay in the cores of dams finds an explanation on
this basis.
When it comes to a single particle diffusing through a liquid
impelled by electrical attraction or other force, the above con-
siderations no longer hold and the walls of the pores offer no
serious resistance, the particle moving through the medium as if
it alone were present, without the surrounding network.